A capacitive motion sensor measures a relative motion between two elements, i.e., a moving element and a fixed element, by sensing a variation of electric capacitance between the two elements.
As for the capacitive motion sensor, two types of motion sensors, i.e., a spacing variation motion sensor and an area variation motion sensor, are well known. For the spacing variation motion sensor, a secondary plate moves in a vertical direction with respect to a primary plate and a capacitance between the two plates is mainly dependent on average plate spacing. With small plate spacing, this type of motion sensor is very sensitive to a spacing change. However, the sensitivity decreases rapidly as the spacing gets larger.
For the area variation motion sensor, a secondary plate slides transversely over a primary plate and the capacitance is mainly dependent on the area of a region on which the two plates are overlapped. This type of motion sensor is less sensitive but can measure a broader range of motion than the spacing variation motion sensor.
However, for both spacing and area variation motion sensors, a measurement range is restricted in that resolution bits are numerically restricted. To overcome this limitation of measurement range without loss of resolution, encoder-type capacitive motion sensors have been developed. In an encoder-type capacitive motion sensor, a simple plate pattern is replicated periodically in a linear array so that periodic signals represent a relative motion. Thus, a signal cycle count establishes a coarse position while a signal phase establishes a fine position.
Referring to FIG. 1, there is illustrated a schematic diagram of a conventional encoder-type motion sensor. A part of FIG. 1 shows a perspective view of a primary and a secondary plate. The encoder-type motion sensor of FIG. 1 includes a primary plate 112, a secondary plate 114, an AC signal source 120, a signal conditioning circuit 130, a (peak) counter 140, an AD converter (ADC) 150, a displacement computing processor 160 and a displacement displayer 170. The primary and secondary plates 112 and 114, both conductive and having a pattern replicated periodically in a linear array, are arranged in parallel, the patterns on both of the plates being confronted with each other. A recurrence distance of the pattern of the primary plate 112 and that of the pattern of the secondary plate 114 are substantially identical with each other. The secondary plate 114, moving in a leftward or rightward direction, can be driven by, e.g., an external actuator.
As the secondary plate 114 moves on, the AC signal source 120 sends a plate input signal to the primary plate 112. The signal conditioning circuit 130 receives a plate output signal from the secondary plate 114, measures the capacitance variation between the two plates and sends an electrical signal containing information on the measured capacitance variation to the counter 140 and the AC converter 150. The counter 140 counts a number of peak recurrences of the electrical signal to generate a cycle count that is sent to the displacement computing processor 160. The AD converter 150 detects and digitizes a phase of the electrical signal to generate a fine motion value to be sent to the displacement computing processor 160. The displacement computing processor 160, receiving the cycle count and the fine motion value, computes a displacement of the secondary plate 114 with respect to the primary plate 112 and sends a displacement value representing the displacement of the secondary plate 114 with respect to the primary plate 112 to the displacement displayer 170. Then, the displacement displayer 170, e.g., an LED display or a monitor, displays information on the displacement value.
As shown above, the conventional encoder-type motion sensor of FIG. 1 can measure a fine motion within a large range by combining the cycle count and the fine motion value. However, the conventional encoder-type motion sensor of FIG. 1 has some drawbacks. First, the conventional sensor of FIG. 1, when used alone, can only measure a one-dimensional displacement. Thus, a sensor of this kind, when used alone, cannot measure a two-dimensional displacement. Second, the conventional sensor of FIG. 1 cannot detect a direction of the displacement. That is, the conventional sensor of FIG. 1 cannot detect whether the secondary plate moves in one direction or the other in a reciprocating motion. Finally, the patterns of the plates in FIG. 1 are difficult to be formed into a minute size.